LibWeb: Remove unecessary dependence on Window from DOM and WebIDL

These classes only needed Window to get at its realm. Pass a realm
directly to construct DOM and WebIDL classes.

This change importantly removes the guarantee that a Document will
always have a non-null Window object. Only Documents created by a
BrowsingContext will have a non-null Window object. Documents created by
for example, DocumentFragment, will not have a Window (soon).

This incremental commit leaves some workarounds in place to keep other
parts of the code building.

Userland/Libraries/LibWeb/DOM/Position.h

@@ -8,6 +8,7 @@
 #pragma once
 
 #include <AK/RefPtr.h>
+#include <LibJS/Heap/Handle.h>
 #include <LibWeb/DOM/Node.h>
 #include <LibWeb/Forward.h>